Rechargeable batteries are more
economical than the alkaline type
By MUNGAI KIHANYA
The Sunday Nation
Nairobi,
14 October 2007
Last week, Oscar Omondi asked several questions about
digital cameras, the last of which was to do with their batteries. He
wanted to know, “which is more economical - using two size AA batteries
or rechargeable Lithium Batteries?”
By “size AA”, I will assume that Oscar is referring
to the long-life Alkaline batteries and not the ordinary Zinc-carbon
type. The latter would only survive a few minutes in a digital camera! I
will also assume that he is talking about the Nickel Metal-Halide
(NiM-H) rechargeable and not Lithium-ion kind.
It is important to note that Lithium batteries CANNOT
be used in place of the alkaline type. The reason is that Lithium cells
produce about 3.6 volts while the alkaline gives only 1.5V. If you fit
Lithiums in a gadget designed to run on alkalines, it will go “poof!”
and burn.
However, NiM-H batteries produce about 1.2V and
therefore, they can be used as a replacement for alkaline cells. For
that reason, we shall compare the “economics” of using alkaline versus
NiM-H.
Now, the service life of a battery is determined by
the energy capacity, that is, how much energy it contains. The capacity
is quoted in ampere-hours (Ah). This translates to the number of hours
that the battery can maintain a current of one ampere at the rated
voltage. Car batteries, for example, have capacities of about 50Ah to
100Ah at 12 volts.
However, the capacity of the small batteries is
usually quoted in milli-ampere-hours (mAh). One mAh is much smaller than
an Ah – there are 1,000mAh in one Ah.
Alkaline batteries have capacities of between
2,500mAh and 3,000mAh when used in low consumption devices.
But when fitted in a gadget that requires high power (like a
digital camera), the capacity drops to about 600mAh.
Rechargeable NiM-H batteries have a more stable
performance. They have a capacity of between 1,500mAh and 2,500mAh –
even when the gadget consumes high power.
Thus from one point of view, the rechargeables are
better than the alkalines. They last about three times longer before
needing a recharge – and remember, the alkalines are thrown away after
use while the NiM-H are just recharged.
But how much does recharging cost?
A typical charger consumes about 25W and takes about
5 hours to “fill-up” the batteries. This works out to 125watt-hours of
power, or 0.125kWh. The kilo-watt-hour (kWh) is the standard unit of
selling power and, at today’s rates, it costs about Sh10.
Therefore, recharging a pair of NiM-H batteries would
cost about Sh1.25 while replacing the alkaline type costs about Sh200!
Clearly, the NiM-H win again on the second point.
How about the purchase costs? A pack of four NiM-H
batteries plus a charger costs between Sh2,000 and Sh3,000 (depending on
make) while a similar number of alkalines (WITHOUT charger, of course!)
goes for about Sh300 only.
Thus ten sets of alkaline batteries will buy one
NiM-H pack. But, the energy in ten alkalines is equivalent to only three
charge cycles of the NiM-H – that is only Sh3.75 in charging cost.
Clearly, then, rechargeable batteries are more
economical than the AA alkaline type.
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